Lithium ion secondary batteries are small, lightweight and high-capacity batteries. Since lithium ion secondary batteries were developed in 1991, they have been widely used as electric sources of portable devices. With the recent remarkable progress of electronic, communication and computer technologies, devices, such as camcorders, cell phones and notebooks, have been introduced into the market and there has been increased demand for lithium ion secondary batteries as power sources for driving the portable digital communication devices.
In particular, a number of studies on power sources for electric vehicles in which an internal-combustion engine is hybridized with a lithium secondary battery have been actively undertaken in the United States, Japan, Europe and other countries. Lithium ion secondary batteries have been considered for use as large-size batteries for electric vehicles, due to their energy density, but are still undergoing development. Particularly, nickel-hydrogen batteries are preferred in terms of safety. The most urgent tasks associated with the use of lithium ion secondary batteries are high price and poor safety.
In particular, cathode active materials, such as LiCoO2 and LiNiO2, generally used at present, have the drawbacks that they do not have a stable crystal structure due to lithium deintercalation during charging, causing poor thermal characteristics.
That is, when an overcharged battery is heated to 200˜270° C., a rapid change in the structure of the battery takes place, which induces a reaction releasing oxygen present in the lattice (J. R. Dahn et al., Solid State Ionics, 69, 265 (1994)).
Commercially available small-size lithium ion secondary batteries use LiCoO2 as a cathode material and carbon as an anode material. LiCoO2 is superior in terms of stable charge-discharge characteristics, high electronic conductivity, superior stability and plateau discharge voltage characteristics, but is disadvantageous in terms of insufficient cobalt deposits, high price and human toxicity. For these reasons, development of novel cathode materials is needed.
LiNiO2, which has a layered structure like LiCoO2, has a high discharge capacity, but is difficult to form into a pure layered structure. Further, since LiNiO2 is transformed into LixNi1−xO having a rocksalt-type structure due to the presence of highly reactive Ni4+ ions after discharge while releasing a large quantity of oxygen, it has some problems of short life and poor thermal stability. Accordingly, LiNiO2 is currently limited in commercial applications.
To solve these problems, there have been attempts to shift the temperature at which heat emission is initiated to a slightly higher temperature by replacing a part of nickel atoms in LiNiO2 with transition metal elements, or to broaden the exothermic peaks for the purpose of preventing sudden heat emission (T. Ohzuku et al., J. Electrochem. Soc., 142, 4033 (1995), Japanese Patent Laid-open No. Hei 9-237631). However, satisfactory results could not be achieved.
Further, LiNi1−xCoxO2 (x=0.1˜0.3) wherein a part of nickel atoms in LiNiO2 are replaced with cobalt atoms shows superior charge-discharge characteristics and cycle characteristics, but fails to solve the problem of poor thermal safety.
Much is known regarding the compositions and preparation processes of Li—Ni—Mn-based composite oxides in which a part of Ni atoms are replaced with thermally safe Mn atoms and Li—Ni—Mn—Co-based composite oxides in which a part of Ni atoms are replaced with Mn and Co atoms.
For example, Japanese Patent Laid-open No. Hei 8-171910 discloses a method for preparing LiNixMn1−xO2 (0.7=x=0.95) as a cathode active material by mixing an alkali solution with a mixed aqueous solution of Mn and Ni to co-precipitate the Mn and Ni, mixing the co-precipitated compound with lithium hydroxide, and calcining the mixture.
Further, Japanese Patent Laid-open No. 2000-227858 suggests a method for preparing a cathode active material based on the new concept that Mn and Ni compounds are uniformly dispersed on the atomic level to form a solid solution, rather than the concept that LiNiO2 or LiMnO2 is partly replaced with transition metals.
However, LiNi1−xCoxMnyO2 (0<y≤0.3) disclosed in European Patent No. 0918041 and U.S. Pat. No. 6,040,090 shows improved thermal stability as compared to conventional materials composed of Ni and Co only, but it has difficulties in commercialization due to the reactivity of Ni4+.
Further, LiaCobMncMdNi1−(b+c+d)O2(M=B, Al, Si. Fe, Cr, Cu, Zn, W, Ti or Ga) wherein Co, Mn and other metals are used in place of Ni is described in European Patent No. 0872450 A1 and B1. However, this material still has the problem of poor thermal safety.